1. Field of the Invention
The present invention relates to a color image forming apparatus suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
Conventionally, in an optical scanning device such as a laser beam printer (LBP), a light source unit modulates optically a light beam according to an image signal and emits the light beam, which is periodically deflected by a deflection unit that includes a rotation polygon mirror. Then, the deflected light beam is condensed so as to form a light spot on a surface of a photosensitive recording medium (photosensitive drum) via an imaging optical system having an fθ characteristics, whereby the surface of the photosensitive recording medium is scanned with the light beam to record an image.
The above-mentioned optical scanning device is provided with a synchronization detection element (synchronous sensor) as an optical detector for adjusting a timing for starting image formation on the surface of the photosensitive drum before scanning the surface of the photosensitive drum with the light spot.
This synchronous sensor receives a light beam for synchronous detection (synchronous light beam) that is a part of the light beam deflected for scanning by the deflection unit. Then, a signal for synchronous detection is detected from an output signal of the synchronous sensor, and the start timing for recording the image on the surface of the photosensitive drum is adjusted based on the signal for synchronous detection.
As one type of such image forming apparatus, “tandem type color image forming apparatus” has been proposed conventionally in various manners (see Japanese Patent Application Laid-Open No. 2002-250880).
FIGS. 12A and 12B are schematic diagrams of main part of the tandem type color image forming apparatus disclosed in FIG. 1 of Japanese Patent Application Laid-Open No. 2002-250880.
As illustrated in FIGS. 12A and 12B, the tandem type color image forming apparatus includes units for forming electrostatic latent images on photosensitive drums 4K, 4C, 4M, and 4Y by scanning with light, and developing units for developing the formed electrostatic latent images, which are arranged in a “tandem type” manner as one unit in one direction. Then, the electrostatic latent images corresponding to yellow (Y), magenta (M), cyan (C), and black (BK) color component images are formed separately on the four photosensitive drums 4K, 4C, 4M, and 4Y. Further, those electrostatic latent images are developed with toner of corresponding colors so that toner images of different colors are obtained on the individual photosensitive drums 4K, 4C, 4M, and 4Y. Then, the toner images of four colors are transferred onto the surface of the same transfer paper and are fixed so that a color image is obtained.
Image quality that is required to the image forming apparatus (printing apparatus) utilizing an electrophotography process has been upgraded year by year. In particular, color displacement is an important issue for image quality in the color image forming apparatus.
One of causes of the color displacement is write start misregistrations of individual colors on the photosensitive drums. This is caused by a mounting error of a component in the apparatus, a shape error of a cabinet itself to which the components are mounted, or the like. As a result, a relative deviation occurs among write start timings of the individual colors, and hence write start positions of the individual colors are also deviated.
In order to decrease a deviation amount of the write start positions, it is useful to increase a focal length of an optical system for synchronous detection so that an influence (sensitivity) of a synchronous detection error affecting the imaging optical system is reduced.
If this method is adopted, however, the following problem may occur in case of the color image forming apparatus including a plurality of optical scanning devices having the same structure like the above-mentioned conventional example.
If a monochrome image is output, only the optical scanning device for black color is operated. For this reason, the optical scanning device for black color is generally used most frequently among the optical scanning devices of four colors for forming a color image. A difference of the frequency of use can be considered as a difference of an occurrence frequency of disturbance such as a temperature rise or vibration during the operation. As a result, therefore, the synchronous detection error in the optical scanning device for black color becomes the largest among synchronous detection errors in the optical scanning devices for colors other than the black color.
On the other hand, in these years, there is also a need of decreasing the focal length of the synchronous detection optical system so as to respond a request for downsizing the optical scanning device. In this case, the influence of the synchronous detection error may increase, and hence a deviation amount of the write start position may increase.